Superconductive magnetic bearings of the type mentioned and already known include those comprising a stationary bearing portion having an annular superconductor unit provided on a stationary portion, and a rotatable bearing portion having an annular permanent magnet unit provided on a rotatable portion so as to be opposed to the superconductor unit. Such superconductive magnetic bearings further include superconductive magnetic bearings of the radial type wherein the two bearing portions are opposed radially of the bearing, and those of the axial type wherein the two bearing portions are opposed axially of the bearing.
FIG. 1 shows an example of conventional superconductive magnetic bearing of the radial type.
With reference to FIG. 1, indicated at 1 is a stationary portion in the form of a shaft, and at 2 a rotatable portion in the form of a hollow cylinder and rotatable around the stationary portion 1. The stationary portion 1 is provided with a stationary bearing portion 3, and the rotatable portion 2 with a rotatable bearing portion 4.
The stationary bearing portion 3 has a superconductor unit 5 in the form of a hollow cylinder. As shown in FIG. 2, the superconductor unit 5 comprises a plurality of superconductor bulks 6 in the form of circumferentially divided segments of a hollow cylinder. Each of the superconductor bulks 6 comprises a superconductor of the second kind having fine normally conductive particles uniformly incorporated therewith. The superconductor bulks 6 are cooled as with liquid nitrogen.
The rotatable bearing portion 4 has two hollow cylindrical permanent magnet units 7, 8 arranged side by side axially of the bearing, and three annular yokes 9 of magnetic material arranged between the adjacent end faces of the two magnet units 7, 8 and over the other end faces thereof. Although not shown in detail, each permanent magnet units 7 (8) comprises a plurality of permanent magnet bulks 10 (11) in the form of circumferentially divided segments of a hollow cylinder. The permanent magnet units 7, 8 each have magnetic poles at axial (upward and downward) opposite ends thereof. The adjacent ends of the two permanent magnet units 7, 8 have the same polarity. In this case, the upper unit 7 has an N pole at its upper end and an S pole at its lower end, and the lower unit 8 has an S pole at its upper end and an N pole at its lower end.
The upper permanent magnet unit 7 sets up a magnetic field indicated by an arrow A in FIG. 1 between the magnet unit 7 and the upper portion of the superconductor unit 5. Similarly, the lower permanent magnet unit 8 sets up a magnetic field indicated by an arrow B in FIG. 1 between the magnet unit 8 and the lower portion of the superconductor unit 5. When the superconductor unit 5 is brought into a superconducting state by cooling, the magnetic fields penetrating into the unit 5 are captured at (pinned to) normally conductive portions (pinning points) of the normal conductor particles in the interior of the unit 5, and the rotatable bearing portion 4 is supported by this pinning effect with respect to the axial and radial directions relative to the stationary bearing portion 3.
In the permanent magnet units 7, 8 of the superconductive magnetic bearing described above, the permanent magnet bulks 10, 11 are circumferentially uniform in magnetic field, but the boundaries between adjacent magnet bulks 10, 11 are not uniform in magnetic field. The yokes 9 are used to diminish such circumferential unevenness of magnetic fields, and the presence of the yokes 9 enables the permanent magnet units 7, 8 to set up substantially uniform magnetic fields. Accordingly, the magnetic fields remain unaltered to produce no resistance to rotation despite the rotation of the magnet units 7, 8 with the rotatable portion 2. The circumferentially uniform magnetic fields are captured at the pinning points of the superconductor unit 5 as described above.
On the other hand, the superconductor unit 5 has the problem that since the unit 5 is divided into a plurality of superconductor bulks 6 as arranged circumferentially thereof, the magnetic fields set up by the unit 5 are uneven with respect to the circumferential direction.
A description will be given of the superconductor bulks 6 wherein the magnetic fields provided by the permanent magnet units 7, 8 are captured. As shown in FIG. 2, the magnetic field is captured at a large number of pinning points 12 in each superconductor bulk 6, and a shielding current indicated by an arrow C flows around the point 12. The magnetic field set up by this shielding current becomes a magnetic field captured by the bulk 6. Inside the bulk 6, the pinning points 12 are uniformly distributed with respect to the circumferential direction, so that the magnetic fields captured are uniform. At the portion of boundary between adjacent superconductor bulks 6, the distribution of pinning points 12 is uneven, and the magnetic fields are also uneven. When this is observed macroscopically, the shielding currents around the pinning points 12 which are uniformly distributed offset one another, whereas at the boundary between superconductor bulks 6, the shielding currents around the outermost pinning points 12 are not offset, with the result that when the bulk 6 is observed in its entirety, a shielding current flows as indicated in a broken line D in FIG. 2. Since such shielding current flows through every bulk 6, the superconductor unit 5 becomes uneven in magnetic field with respect to the circumferential direction. If the magnetic fields set up by the unit 5 become circumferentially uneven, the permanent magnet units 7, 8 are subjected to varying magnetic fields when rotating with the rotatable portion 2, producing eddy currents in the units 7, 8 and giving rise to a rotation loss,
Eddy currents also occur in the yokes 9 owing to the circumferential unevenness of magnetic fields in the superconductor unit 5.
An object of the present invention is to overcome the above problem and to provide a superconductive magnetic bearing which is diminished in the eddy currents occurring in the permanent magnet units and in the yokes due to the unevenness of magnetic fields set up by the superconductor unit to ensure a reduced rotation loss.